1. Field of the Invention
This invention relates to a process of producing sulfuric acid from SO.sub.2 -containing gases comprising a catalytic reaction to convert SO.sub.2 to SO.sub.3, a multi-stage cooling of the SO.sub.3 -containing gases before an absorption of SO.sub.3 in concentrated sulfuric acid, a transfer of heat from the sulfuric acid circulated through the absorber to a cooling fluid by an indirect heat exchange, a circulation of the cooling fluid, and a transfer of heat from the cooling fluid cycle to a consumer by an indirect heat exchange.
2. Discussion of Prior Art
V.sub.2 O.sub.5 catalysts are employed in practice for the catalytic reaction by which SO.sub.2 is converted to SO.sub.3. Because the reaction is exothermic, it raises the temperature of the gases. The absorption of the resulting SO.sub.3 in concentrated sulfuric acid is also an exothermic process and the heat of absorption which is liberated must be dissipated. For reasons of heat economy, it is important to utilize the surplus heat which becomes available. As that heat becomes available in part at relatively low temperatures, the economically useful utilization of that heat is difficult. Additionally, it must be ensured that the temperature of the SO.sub.3 -containing gases will not drop below the dew point temperature before said gases enter the absorber because even dried contact process gases still have a small moisture content. This applies to the interstage absorption and to the final absorption. It is known that the SO.sub.3 -containing gases leaving the first contact process stage can be cooled in two stages before the interstage absorption. The first stage involves a heat exchange with the colder gases leaving the interstage absorption, and the second stage involves a heat exchange with feed water (German Patent Specification No. 1,186,838, German Patent Specification No. 1,567,672).
From German Patent Specification No. 2,529,709 and Laid-open German Applications Nos. 2,529,708 and 2,824,010 (U.S. Pat. No. 4,276,116) it is known to dissipate heat from the acid cycles of the absorbers by an indirect heat exchange with dilute sulfuric acid or phosphoric acid. The heat exchange may be effected through the intermediary of a closed cycle of a cooling fluid consisting of water, oil or other fluids. A relatively high temperature of the cooling fluid involves a relatively high expenditure regarding the acid-cycle heat exchanger. When the SO.sub.3 -containing gases are cooled in two stages before entering the absorber, the temperature may drop below the dew point temperature if the heat of the gases is used to a large extent to heat the cooling fluid.
It is an object of this invention to increase (in the closed heat transfer cycle between the absorber acid and the consumer the temperature of the liquid cooling fluid) or to achieve approximately the same temperature with a lower expenditure in such a manner that surplus heat from the contact process system is utilized and it is ensured that the temperature of the SO.sub.3 -containing gases does not drop below the dew point temperature.